1. Field of the Invention
This invention relates to a semi-insulating aluminum nitride sintered body having small variations in volume inherent resistivity values and a small resistance temperature coefficient, and particularly, a semi-insulating aluminum nitride sintered body preferably used in such utilities as members for removing static electricity and an electrostatic chuck.
2. Description of the Prior Art
Among various ceramic sintered bodies, aluminum nitride sintered bodies have a high heat conductivity, and are used in a field which particularly requires a high heat radiatability. Especially in recent years, such sintered bodies have been used as the members for removing static electricity in separating claws for printers, tape guides for guiding magnetic tapes, and conveying arms or vacuum chucks with which an apparatus for producing electronic component parts or a semiconductor apparatus is provided. Furthermore, such sintered bodies have been used as a dielectric layer of the electrostatic chucks for supporting and conveying various articles by electrostatic adsorption forces.
Since the aluminum nitride sintered products are a highly insulated material (at least about 10.sup.16 .OMEGA..multidot.cm at room temperature), their inherent resistivity value (to be referred to simply "resistivity value") should be adjusted depending upon uses.
For example, when the aluminum nitride sintered product is used as a member for removing static electricity, its resistivity value must be adjusted to about 10.sup.4 to 10.sup.8 .OMEGA..multidot.cm.
When it is used as a part comprising an electrostatic chuck, the resistivity value must be adjusted to 10.sup.8 to 10.sup.12 .OMEGA..multidot.cm, especially about 10.sup.8 to 10.sup.11 .OMEGA..multidot.cm at a temperature in the range of room temperature to 100.degree. C., especially -30 to 200.degree. C. As means for adjusting such a resistivity value, it is known that by including an electric conducting material into the aluminum nitride sintered body, its resistivity value can be decreased.
Japanese Laid-Open Patent Publication No. 8089/1994, for example, discloses an aluminum nitride sintered body in which TiN is included in an amount of 5% by weight or below as an electric conducting material. This sintered body has a resistivity value of 10.sup.8 to 10.sup.13 .OMEGA..multidot.cm in a room temperature region. In the following explanation, the volume inherent resistivity value (resistivity value) means a value at room temperature (25.degree. C.) unless specified otherwise.
However, since many of electric conducting materials such as TiN used to decrease the resistivity value have very small volume resistivity value of not larger than 10.sup.2 .OMEGA..multidot.cm, and in addition, exist in the form of particles in the sintered body, aluminum nitride sintered bodies containing such an electric conducting material have a defect in that variations in resistivity value are large. Especially, it is difficult to produce aluminum nitride sintered bodies which have a resistivity value of about 10.sup.4 to 10.sup.12 .OMEGA..multidot.cm required in a member for removing static electricity or electrostatic chucks with good reproducibility, and there is a problem in that the yield is poor.
For example, if the content of the electric conducting material is increased, the electric conducting materials are united with each other, and an abrupt decrease occurs in the resistivity value. Therefore, variations in the resistivity values become large, and it is difficult to obtain sintered bodies having an intended resistivity value. Furthermore, if the content of the electric conducting material is reduced, it is impossible to form a sufficient conductive passage within the inside of the aluminum nitride sintered body, and it is difficult to lower the resistivity of the sintered body. In addition, since a sufficient conductive passage is not formed, the resistivity values vary greatly due to changes in temperature. As a result, inconvenience occurs in that an aluminum nitride sintered body cannot be used effectively this way in a narrow temperature range.